Comparison of open-atmosphere nitriding on various metal surfaces triggered by a focused pulsed laser irradiation

Abstract

This study aimed to further the understanding of open-atmosphere nitriding phenomena on a metal surface triggered by focused pulse Nd:YAG laser irradiation. This phenomenon produces a nitride layer of several micrometers on the Ti surface but not on other metal surfaces, such as Fe. In this study, a similar laser treatment was conducted on Ti, Fe, Zr, and Cu surfaces. The key material properties supporting the growth of the nitride layer were examined based on the surface analysis results. The results highlight the importance of the thermal conductivity of the metal substrate and the standard free energy of each metal nitride in laser nitriding phenomena. Thermal conductivity is related to the generation of the molten metal pool, which predominates the intake of nitrogen gas from the surrounding gas. For example, a Cu surface with a high thermal conductivity of approximately 4 × 102 W·m−1·K−1 scarcely takes in nitrogen. The standard free energy is important for the formation of nitride compounds during solidification. For instance, the nitrogen on the Fe surface was degassed immediately because of the high standard free energy of Fe-nitride (e.g., Fe4N with 3.8 kJ·mol−1 at 298 K); consequently, the nitrogen content was notably diminished. Open-atmosphere nitriding phenomena are complex; however, the findings from this study provide valuable insights into the process.